It is possible for kube config file to contain a context that refers to cluster
or user configurations that don't exist. I don't know how, but my kubeconfig
file does, and even when I clean it up, this frequently happens. These contexts
are old contexts that I don't use anymore, so they are safe to ignore. kubectl
just ignores them. Skuber should do the same.

This adds support for async auth providers. There are many use cases for this,
for example, authenticating with Kubernetes in AWS but outside of EKS, you need
to make API calls to get a frequently rotated token. This allows that token to
be looked up in a non blocking way.

This also prepares the way for better support for reloading tokens in
Kubernetes itself given that they are now frequently rotated.

Fixes #350

This implements token reloading for in cluster config. It's built on #424 so
that needs to be reviewed/merged first.

Also fixed a resource leak where the input streams for reading the files for
in cluster config were not being closed. This was necessary to fix given that
we're now frequently loading those streams.

Also removed legacy beta versions of apps group